FIELD EFFECTS ON THE DIELECTRIC PROPERTY OF YBCO BICRYSTAL GRAIN-BOUNDARY JUNCTIONS

Electric field effects on YBCO boundary Josephson junctions combined with MIS (Metal, Insulator, Superconductor) structures were studied. Current steps and hysteresis recognized in current-voltage curves were strongly affected by the field. The current steps were attributed to the self-excited resonances, i.e., Fiske steps. Both the effects were interpreted by the field dependence of the effective dielectric constant of the grain boundary which has a certain contribution from the large dielectric constant of SrTiO3 substrates. The effective dielectric constant normalized with barrier thickness of the junction was estimated to be about 40 nm(-1) for the zero gate voltage and was decreased by a factor of 2 with the electric field of +/-16kV/cm. This feature is assumed adequate for tuning the phase velocity in the bicrystal grain boundaries.